Recieving Error 500


i installed my Nextcloud.
I am accessing it via a DynDNS Strato-Subdomain.
My FritzBox is doing the Dynamic Update.

So i have another Subdomain. So i added a reverse Proxy entry to my default conf file.
Now when i try to access my Domain i am recieving an Error 500:

Here is the apache log:
[Wed Apr 19 15:26:37.789619 2017] [:error] [pid 2547] [client 192.168.0.x:4760] PHP Fatal error: Uncaught Error: Call to a member function getLogger() on null in /var/www/nextcloud/index.php:61\nStack trace:\n#0 {main}\n thrown in /var/www/nextcloud/index.php on line 61

Somebody has an idea where he Problem comes from?